I don't know if this has been mentioned, but I prefer the "Snuggle Safe" heating disc. This is what we used to use in the vet clinic for patients that needed to be kept warm. It is microwaveable, holds heat very nicely and for a long time and won't burn skin. You can tuck it in any zipper bed or just put it on top of or under blankets etc. I sware by it and when I finally breed a litter it will be what I use. Here is a link.http://www.jefferspet.com/ssc/produ...1T6N6SM6EM7QBFQA2&pf_id=998&cmkw=snuggle safe